A vertical structure, bipolar power transistor comprises a substrate of the N+ type overlaid by an N- epitaxial layer wherein an isolation well of the P type is formed whose ends are flush with the device surface.
The substrate provides the transistor collector, while the isolation well, contacted at the device surface, provides the transistor base.
The emitter region usually consists of one or more regions of the N+ type formed within the P-well and being contacted at the surface by conduction paths referred to as the metallizations, or briefly the "metals."
The current through a power transistor is sensed by measuring the voltage drop across a resistor, generally provided in series with the terminal, the voltage drop being due to the current flowing through the transistor. This resistor, known as the "sensing" resistor, is usually integrated to the device in an area close to that of the power transistor, or alternatively, formed by modifying somewhat the transistor structure, as shown in FIGS. 1, 2 and 3.
FIG. 1 is a plan view of a prior power transistor incorporating a sensing resistor. Also shown in the figure are the base and emitter metallizations 40 and 41, respectively, in contact with corresponding buried regions, not illustrated in the figure.
A sensing resistor 42 is formed from a metallization section placed in series with the emitter contact 43. A current is caused to flow through the sensing resistor 42 in the metallization section which is proportional to the total emitter current and produces a voltage drop that can be easily sensed, e.g., by means of two contacts 44 and 45 as shown in the Figure.
FIG. 2 is a plan view of another prior power transistor, also incorporating a sensing resistor. A base metallization 20 and emitter metallization 21 contact corresponding buried base and emitter regions, not illustrated in the Figure.
The emitter region is split into plural zones, referred to as the emitter "fingers" on account of their peculiar shape and which are connected together by the emitter metal 21. The emitter current is measured here by measuring, through contacts 24 and 25, the difference of potential across the metallization of an emitter finger 22.
FIG. 3 is a plan view of a further prior power transistor to which a surface diffused sensing resistor RS has been incorporated. In particular, in order to take a measurement of emitter current, the difference of potential across a diffused resistor RS, being placed in series with an emitter portion and similarly doped, is measured. Two base metallizations 30 and an emitter metallization 31 contact respective base and emitter regions.
The active region 32 of the emitter is contacted by means of a first emitter contact C1 and a metallization 34, while the resistor RS is contacted by means of a second emitter contact C2 and a metallization 35. The difference of potential across the resistor RS, as due to the current flowing from the emitter active region to the second emitter contact C2, is measured between the ends of said metallizations 34 and 35.
Thus, in prior integrated structures, the sensing resistor has been formed at the surface by having metallization or diffusion areas modified in or added to the structure of the power transistor. These techniques invariably result in an increase of the area occupied by the power transistor.